Get pose bones which were animated in a strip
The container for fcurves are actions. So you need to find the corresponding action for the strip.

I.e.

track = C.active_object.animation_data.tracks[0]
#get the action from the first strip
action = track.strips[0].action

for fcu in action.fcurves:
    print(fcu.data_path, fcu.array_index)